1. Field of the Invention
The invention relates generally to alpha-alumina inorganic membrane supports and more particularly to controlling porosity, pore distribution and strength characteristics of alpha-alumina inorganic membrane supports via control of alumina and/or pore former particle sizes and other process variables.
2. Technical Background
In the field of membrane separations, thin porous materials deposited on porous supports are widely used for micro-filtration or ultra-filtration of liquid media and for gas separation. The porous support functions to provide mechanical strength for the thin porous materials.
Porous ceramic supports can be deposited with inorganic coatings to form a membrane structure for use in, for example, filtration and separation applications in, for example, the environmental, biological, food and drink, semiconductor, chemical, petrochemical, gas and energy industries. These industries often require purified gas/vapor or purified liquid whose source is a mixed feed stream composed of different gas and/or liquid/particulate combinations. Specific examples include purification and separation of hydrogen gas, sequestration of carbon dioxide gas, filtration of oil/water mixtures, wastewater treatment, filtration of wines and juices, filtration of bacteria and viruses from fluid streams, separation of ethanol from biomass, production of high purity gas and water for the semiconductor and microelectronics industry.
The membrane support functions to provide a high geometric surface area packing density for film/coating deposition and, at the same time, it is advantageous if the support has high permeability, strength, chemical stability, thermal stability and structural uniformity.
A monolithic inorganic membrane product concept and macroscopic design parameters are described in detail in commonly owned US Patent Application Publication 2006/0090649. This design has the advantage of high surface area packing density and geometric simplicity for easy engineering as compared to conventional designs. In this design configuration, it is advantageous that the support have high permeability and high strength.
Commonly owned EP Patent No. 0,787,524 relates to a mullite membrane support design. The mullite membrane support is configured as a honeycomb of 0.2 μm average pore size and is used to fabricate a membrane module.
Commonly owned U.S. Pat. No. 5,223,318 relates to the making of titania substrates for membrane supports.
High purity α-alumina is known to have high chemical stability in acid, base and other reactive environments and also has high thermal and hydrothermal stability. High purity α-alumina is used in research on various inorganic membranes, and is a preferred material from which to make membrane supports. Alumina membranes, generally in a single tube form, have been developed and used for isotopic separation of uranium for nuclear reactor applications. Abe, Fumio; Mori, Hiroshi. “Inorganic porous membranes” (NGK Insulators, Ltd., Japan) Jpn. Kokai Tokkyo Koho (1990), relates to a method of using a glass binder to make an α-alumina support of a multi-channel structure. Pall Corporation provides alumina supports in single tube and multi-channel form under the product name Membralox®. However, these existing multi-channel products have large channel sizes (>2 mm) and thus provide low surface area packing density as well as low porosity (<36%).
There is a need for an α-alumina support in a monolith structured form having high purity, large pore size, high porosity and uniform pore distribution facilitating a membrane support capable of being used for a variety of membrane filtration applications. The high purity of the α-alumina support would be useful in maintaining chemical stability, since impurities generally make the alumina more reactive. The large pore size and the high porosity would be useful in, for example, providing high permeability and high thermal stability. At the same time, the pore structure and distribution should be well balanced in order to maintain mechanical strength and structural uniformity.